Understanding carrier screening: Essential insights for family building

What is carrier screening?

Carrier screening is a test to see if someone is carrying gene mutations for certain genetic disorders. It’s a proactive step that many people consider when planning to start a family. 

But navigating the information about carrier screening can sometimes feel overwhelming. Keep reading our article to get the lowdown on carrier screening and see if it’s right for you.

What’s involved in carrier screening?

Carrier screening is typically done through a simple blood test or saliva sample. The goal of the test is to identify carriers who may not exhibit symptoms but could pass the mutated gene to their children.
What does a positive or negative result mean?

A positive result means that someone carries a gene mutation for a specific disorder. However, it’s important to understand that being a carrier doesn’t necessarily mean their future children will develop the disorder. It simply means their relatives are at an increased risk of carrying the same mutation.

If both you and your partner carry the same recessive genetic condition, there’s a 25% chance of passing it on to the child in each pregnancy. 

If a woman carries an X-linked condition, her probability of passing it on to her child is as much as 50% per pregnancy.

A negative result indicates that no known mutations were detected for the disorders that were tested, but it doesn’t eliminate the risk entirely.

Why is carrier screening often recommended by fertility clinics?

IVF clinics and healthcare providers often recommend carrier screening to individuals or couples who are planning to have children. The reason for this is to provide people with information about their genetic risk factors and their potential reproductive outcomes. 

Equipped with this knowledge, people are more empowered to make informed decisions related to family planning, as well as preconception or prenatal treatments. 

Carrier screening is highly recommended in the following cases:

• If you or your family has a history of fragile X-related disorders or intellectual disabilities, or if you have a history of primary ovarian insufficiency (menopause prior to the age of 40), you should consider fragile X premutation carrier screening
• Couples who are closely related (consanguineous) should consider genetic screening and counseling to understand their higher chances of having children with recessive genetic conditions.
  

Most of the genetic conditions included in genetic carrier screening are rare. However, when you’re screened for many conditions at once, it’s quite common to learn you are a carrier for one or more of them. 

The more conditions you’re screened for, the higher the chance you’ll be a carrier for at least one. For example, when using a panel screening for 274 conditions, approximately 7 in 10 people will learn they are carriers for at least one condition. 

In the unlikely event that both intended biological parents are carriers for the same condition(s), it’s vital to get genetic counseling. A genetic counselor can give you a clear understanding of what this might mean for your future children. Together, you can discuss possible interventions in the hopes of decreasing the risk.

People who are at increased risk of passing on a disease to their children and undergo IVF may choose to have preimplantation genetic testing (PGT). 

PGT is a process used to test embryos for a specific genetic condition. It allows your physician to select and transfer only the embryos that aren’t expected to be affected by the condition. 

If IVF with PGT isn’t right for you, genetic testing can also be performed during pregnancy through either chorionic villus sampling (CVS) in the first trimester or amniocentesis in the second trimester. 

CVS and amniocentesis both provide highly accurate genetic test results for the fetus – so you can find out if the condition was passed down or not. The results may be important for pregnancy management or planning medical treatment for the birth. 

Finally, some people choose to use an egg or sperm donor who is not a carrier for the same condition in order to greatly reduce the chances of passing a genetic disorder onto their child.

Most common genetic disorders

Carrier screening typically looks for a range of genetic disorders. These include but are not limited to:

• Cystic Fibrosis (CF): A genetic disorder affecting the lungs, pancreas, digestive system, and other organs. Carrier screening for CF is recommended for everyone before getting pregnant. 
• Sickle Cell Disease (SCD): Inherited red blood cell disorders involving abnormal hemoglobin production. Screening with a complete blood count and hemoglobin electrophoresis is recommended for everyone before getting pregnant. 
• Thalassemias: Inherited red blood cell disorders affecting hemoglobin production. Screening with a complete blood count and hemoglobin electrophoresis is recommended for everyone before getting pregnant. 
• Tay-Sachs Disease: A rare genetic disorder affecting the nervous system. 
• Fragile X Syndrome: An X-linked disorder that causes autism or autism spectrum disorder behaviors with intellectual disabilities. 
• Spinal Muscular Atrophy (SMA): A genetic disorder affecting motor neurons in the spinal cord causing muscle weakness and atrophy and a leading genetic cause of infant mortality. Carrier screening is recommended for everyone, regardless of ethnicity.
• Canavan Disease: A rare genetic disorder affecting the brain and nervous system.
• Familial Dysautonomia (FD): A genetic disorder affecting the nervous system.
• Familial Mediterranean Fever (FMF): Characterized by recurrent fever and inflammation.
  

Prevalence of common genetic diseases

How common genetic disorders are can vary a lot depending on various factors, like where in the world you live, your ethnicity, and demographics.

1. Cystic Fibrosis (CF): Occurs in approximately 1 in 2,500 to 3,500 newborns in the United States, with a higher prevalence in the non-Hispanic white population.
2. Sickle Cell Disease (SCD): Affects approximately 1 in 300 African American newborns in the United States. Sickle cell trait is more common in individuals of African, Mediterranean, Middle Eastern, and South Asian descent.
3. Thalassemias: The prevalence varies by type and geographic region, with higher rates in populations where thalassemias are more common. These include people of Mediterranean, Southeast Asian, Middle Eastern, Hispanic, African and West Indian descent.
4. Tay-Sachs Disease: More common in individuals of Ashkenazi Jewish descent, with a carrier frequency of approximately 1 in 30 in this population. Carrier screening is recommended for people of Ashkenazi Jewish, French-Canadian, or Cajun descent. 
5. Fragile X Syndrome: Prevalence is approximately 1 in 3,600 males and 1 in 4,000-6,000 females. Carriers of the Fragile X premutation are also at risk for fragile X-related tremor/ataxia syndrome and primary ovarian insufficiency. Fragile X premutation carrier screening is recommended in females with a family history of Fragile X-related disorders or intellectual disability suggestive of fragile X syndrome. It’s also recommended for females with a personal history of primary ovarian insufficiency.
6. Spinal Muscular Atrophy (SMA): Occurs in approximately 1 in 6,000-10,000 live births. 
7. Canavan Disease: Carrier screening is recommended for individuals of Ashkenazi Jewish descent. Prevalence is approximately 1 in 6,400 to 1 in 13,500 in this population.
8. Familial Dysautonomia (FD): More common in individuals of Ashkenazi Jewish descent. Carrier screening is recommended, with a carrier frequency of approximately 1 in 30 in this population.
9. Familial Mediterranean Fever (FMF): Carrier screening may be recommended for individuals with a family history or of Mediterranean descent, particularly Armenian, Jewish, Arabic, Turkish, and Sephardic descent. 

Understanding carrier screening is an important aspect of preconception and prenatal health. But it’s essential to approach it with an open mind and gather information from reliable sources. 

Discuss carrier screening with your healthcare provider so you can get personalized guidance based on your personal situation, family history, ethnic background, and preferences. Ultimately, the decision to undergo carrier screening is up to you, and you are welcome to make choices for your family that align with your personal values and goals.

• ACOG Recommendations on Carrier Screening for Genetic Conditions. Accessed May 9, 2024: https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/03/carrier-screening-for-genetic-conditionshttps://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/03/carrier-screening-for-genetic-conditions
• Carrier Screening in the Age of Genomic Medicine Accessed May 9, 2024: https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/03/carrier-screening-in-the-age-of-genomic-medicine